Error 500 - wordpress (Internal Server Error)

Wordpress site did not working

Internal Server Error

The server encountered an internal error or misconfiguration and was unable to complete your request.

Log error:

91.200.55.54 - - [23/Dec/2015:17:13:16 +0200] "GET / HTTP/1.1" 500 546 "-" "Mozilla/5.0 (Windows NT 6.1; WOW64; rv:43.0) Gecko/20100101 Firefox/43.0"
[Wed Dec 23 17:13:17 2015] [error] [client 91.200.55.54] SecurityException in Application.cpp:175: Calling user is not webserver user!
[Wed Dec 23 17:13:17 2015] [error] [client 91.200.55.54] Premature end of script headers: index.php

Please reset the file and folder permissions for the domain owner under User Accounts --> fix permissions